A major cause of chronic, and often disabling, back pain is disruption or degeneration of an intervertebral disc. The spine is comprised of bony vertebrae separated by intervertebral discs. Each intervertebral disc connects adjacent vertebrae and forms a joint that allows movement of the vertebral column. An intervertebral disc is generally divided into two regions: the nucleus pulposus and the annulus fibrosus. The nucleus pulposus is a gelatinous-like tissue that lies at the center of the disc and provides a cushion between adjacent vertebrae. The annulus is made up of collagen fibers that form concentric lamellae that surround and contain the nucleus pulposus.
There are many causes of disruption and degeneration of intervertebral discs, which can be broadly categorized as mechanical, genetic and biochemical. Mechanical damage includes herniation in which a portion of the nucleus pulposus projects through a fissure or tear in the annulus fibrosus. Genetic and biochemical causes usually result from changes in the biochemical processes of a disc. Such changes can be attributed to genetic disorders or environmental influences. Degenerative disc condition is commonly caused by a change in the biochemical process of an intervertebral disc. Such degeneration is a progressive process that usually begins with a decrease in the ability of the nucleus pulposus to absorb water. With a loss of water content, the nucleus becomes dehydrated, resulting in a decrease of internal disc hydraulic pressure, and ultimately to a loss of disc height. This loss of disc height can cause the annulus to buckle, eventually resulting in annular fissures and ruptures. Herniation occurs when a rupture leads to protrusion of the nucleus pulposus through the annulus.
Furthermore, disc height plays an important role in the functionality of the intervertebral disc and spinal column, and changes in disc height can have both local and wider effects. On the local (or cellular) level, decreased disc height may result in increased pressure in the nucleus pulposus, which can lead to a decrease in normal cell operation and an increase in cell death and disintegration. In addition, increases in intra-discal pressure may create an unfavorable environment for fluid transfer into the disc, which can cause a further decrease in disc height.
Decreased disc height also results in significant changes in the larger mechanical stability of the spine. With decreasing height of the disc, the facet joints bear increasing loads and may undergo hypertrophy and degeneration. Decreased stiffness of the spinal column and increased range of motion resulting from loss of disc height can lead to further instability of the spine, as well as back pain.
Several disc defects may be treated by implantation of a prosthetic into the nuclear space of the intervertebral disc. Some procedures that may include insertion of a prosthetic into the disc are spinal fusion and disc repair and replacement. Prior to implantation of most prosthesis, a discectomy is often performed to prepare the nuclear space for implantation of the prosthetic and, when spinal fusion is desired, to facilitate bony fusion between the vertebral bodies. Some implantation procedures may require a total discectomy in which the majority (and usually all) of the volume of the nucleus pulposus is removed. Others may require a partial discectomy in which only a portion of the nucleus pulposus is removed.
Traditionally, discectomy procedures are performed with the use of simple manual instruments, such as curettes, which are cupped scrapers with a variety of end configurations, pituitary rongeurs, which are jaw like gripping or cutting members, and rasps, which include a rough surface that is employed to roughen and scrape endplate tissue of adjacent vertebrae. For a typical posterior surgical approach, an incision is made through the back of a patient and access to the disc space is achieved. The manual instruments are then inserted through the access to the intervertebral disc requiring treatment. The curettes and rongeurs are used to cut, tear, and remove nucleus pulposus tissue one piece at a time, and the rasps are utilized to roughen or scrape the endplates of adjacent vertebrae.
There are some significant limitations associated with performing a discectomy with these manual instruments. For example, since the disc tissue is cut and removed a piece at a time, dozens of repeated cycles of insertion and removal of the traditional instruments are required to remove the desired amount of tissue. The repeated cycles increase the risk of associated nerve damage and the amount of trauma to the surrounding tissue. Additionally, guidance of the traditional instruments is largely controlled by the dexterity of the surgeon, and even with the most skilled surgeons, repeated precise placement of such instruments is a challenge. Furthermore, because of the geometric configuration of traditional instruments and the limited work space associated with intervertebral disc procedures, it can be difficult to adequately remove the required amount of material from the nuclear space. This is particularly the case with a unilateral (one of the more preferred) access of the disc space, where the contralateral half of the disc is significantly more difficult to reach. Finally, surgeons typically use traditional instruments without being able to see the tissue being removed. Thus, the surgeon must be able to distinguish nucleus tissue from annulus tissue and bone tissue by “feel.” Thus, if the surgeon has a difficult time distinguishing between these tissues, serious damage can be done to the annulus of the disc or the vertebral bodies.
Other methods and techniques have been developed for performing discectomy procedures. However, these methods and techniques also have limitations and risks associated with their use. Accordingly, there remains a need for improved discectomy devices and methods.